issue with to_date function with sysdate

Question

I saw this question so I've one doubt regarding this question: I am getting different result with the same date on the following query.

SELECT TO_CHAR(to_date(sysdate, 'DD-MON-yy'), 'DAY'),
  TO_CHAR(to_date(sysdate, 'DD-MON-yyyy'), 'DAY'),
  TO_CHAR(to_date(sysdate, 'DD-MON-rr'), 'DAY'),
  TO_CHAR(to_date(sysdate, 'DD-MON-rrrr'), 'DAY')
FROM dual;

output as column-wise:

TUESDAY SUNDAY TUESDAY TUESDAY

please help me, Thanks in advance.

EDIT

I wrote a simple procedure which passing date for finding the day given below:

SET serveroutput ON;
CREATE OR REPLACE
    PROCEDURE simple_test
      (
        date_in         IN VARCHAR2)
                        IS
      v_date DATE       := to_date(date_in,'dd-mon-yyyy');
      v_day VARCHAR2(10):=TO_CHAR(v_date,'day');
    BEGIN
       dbms_output.put_line('the day of given date is '||v_day);
    END;
    /

EXEC simple_test(sysdate);

anonymous block completed
the day of given date is sunday  

EXEC simple_test('01-JAN-2013');

anonymous block completed
the day of given date is tuesday  

why does this happened?

Answer 1

I want to explain why you get different results.

See this sqlfiddle

As it is already said, sysdate is seen as DATE type and you are doing an implicit conversion when

select to_date(sysdate, format) from dual;

because first parameter of to_date should be varchar type the system does:

select to_date(to_char(sysdate), format) from dual;

because your implicit date format is 'DD-MON-YY', your query goes into:

SELECT TO_CHAR(to_date('01-JAN-13', 'DD-MON-yy'), 'DAY'),
  TO_CHAR(to_date('01-JAN-13', 'DD-MON-yyyy'), 'DAY'),
  TO_CHAR(to_date('01-JAN-13', 'DD-MON-rr'), 'DAY'),
  TO_CHAR(to_date('01-JAN-13', 'DD-MON-rrrr'), 'DAY')
FROM dual;

the second to_date, because yyyy is a full thousands year format, goes to '01-JAN-0013' which is 13AD and probably is SUNDAY :)

Answer 2

SYSDATE is already a date. So if you write:

TO_DATE(SYSDATE, 'DD-MON-yy')

you are already doing two conversions, an implict one from date to string and an explicit one from string to date. The implicit one is the problem as you can't specify the date format. (Instead it's taken from the session's current settings.)

So the solution is to get rid of the unnecessary conversion an just write:

SELECT TO_CHAR(SYSDATE, 'DAY') FROM DUAL;